1. Field of the Invention
This invention relates to an improved enhanced oil recovery process. More particularly, the invention relates to such a process for use in subterranean reservoirs containing both a crude oil containing potentially surface active organic acids and an aqueous phase containing water-soluble dissolved divalent metal cations.
2. Description of the Prior Art
There are numerous methods for the enhanced recovery of oil from subterranean reservoirs. Many of these methods involve injection via an injection well of a slug of an aqueous treating solution followed by a slug of drive fluid while producing fluids from the reservoir via a production well. One particular method long recognized by those skilled in the art, involves, in part, injection of a slug of a highly alkaline aqueous solution. U.S. Pat. No. 1,651,311 to Atkinson describes a method for recovering additional oil from a reservoir following primary production by forcing through the reservoir an aqueous solution of a strong alkali, such as soda ash, caustic soda or caustic potash. Although the mechanism by which such an alkaline solution functions as an enhanced recovery fluid is not completely understood, it is believed that in one aspect the treating solution reduces the oil-water interfacial tension in the reservoir by activation by the alkaline material of potentially surface active organic acids naturally occurring in the reservoir crude oil. Also, in some reservoirs, the treating solution may produce a wettability reversal that improves the displacement of oil, i.e., the wettability of the reservoir is reversed from oil-wet to water-wet. Thus, an aqueous drive fluid is more effective in displacing oil from the reservoir.
Various other enhanced oil recovery systems have been suggested. Many reservoirs contain an aqueous phase or brine in addition to oil. It is not uncommon for brine to contain dissolved divalent metal cations such as calcium, magnesium and iron in addition to various other anions and cations. It has been recognized with enhanced oil recovery systems other than those depending primarily on aqueous solutions of alkaline materials, e.g., micellar displacement systems made up of a micellar solution phase and an aqueous phase, that the presence of such dissolved metal cations in the reservoir brine decreases the effectiveness of the enhanced oil recovery system. U.S. Pat. No. 4,036,300 to Holm et al. shows a petroleum recovery process employing an improved micellar improvement system which is effective even in the presence of divalent cations. The micellar displacement system includes a micellar solution phase and an aqueous phase incorporating an aminopolycarboxylic acid chelating agent or the ammonium or alkali metal salt thereof. In one embodiment the micellar displacement system can be prepared, in part, by adding the acid form of the chelating agent to the aqueous phase and then adjusting the pH of this phase to at least about 9 by the addition thereto of a basic material such as ammonium, sodium or potassium hydroxide.
In various treatments of well bores and the surrounding environment other than enhanced oil recovery treatments, it is common to contact the reservoir with an aqueous solution of various materials. In U.S. Pat. No. 3,684,720 to Richardson there is described a method for removing calcium sulfate scale from surfaces such as the bore hole wall, pipe and the formation itself by contacting the same with an inert solvent which is preferably deionized water containing (1) a scale converter selected from ammonium and alkali metal carbonates, bicarbonates, hydroxides, phosphates, oxylates and normally liquid lower alkyl amines and hydroxy substituted derivatives thereof, and (2) a chelating agent such as the ammonium, amine, hydroxylamine and alkali metal salts of alkylenepolyamine polycarboxylic acids, nitrilotriacetic acid and N-2-hydroxyethylamino diacetic acid. The solvent and complexed scale are then removed from the well.
While many of the above described processes have certain advantages, need remains for an enhanced oil recovery method which is more effective in displacing oil for use in reservoirs containing both a residual oil phase which is especially susceptible to displacement by an aqueous alkaline solution and an aqueous phase, i.e., connate water, containing dissolved divalent metal cations which are harmful to many enhanced oil recovery systems, which cations may react with and remove from the enhanced oil recovery composition certain treating agents.
Accordingly, it is a principal object of this invention to provide a method for the enhanced recovery of oil from reservoirs containing a crude oil phase which is especially susceptible to displacement by an aqueous alkaline solution.
It is another object of the invention to provide such a method wherein the crude oil contains potentially surface active organic acids.
It is a further object of the invention to provide such a method wherein the reservoir also contains connate water containing dissolved divalent metal cations.
It is a still further object of the invention to provide such a method for recovery of viscous crude oil.
It is yet another object of the invention to provide such a method employing an aqueous alkaline solution wherein precipitation of divalent metal salts of the alkaline material in the reservoir is minimized.
Other objects, advantages and features of the invention will become apparent from the following description and appended claims.